A film score is original (usually instrumental) music composed and recorded especially for a film. Written by a single composer, though sometimes more, it is designed to fit the film and its story perfectly, adhering to a strict timecode. Film scores were traditionally performed by orchestras, and many still are, but today a film score might feature all manner of sounds and instruments.
A film soundtrack is more a selection of songs chosen to be featured in a film. In most cases they are existing tracks that have been licensed by the film’s producers; in rarer cases films feature original songs, written especially with the film’s story in mind.
The word ‘Soundtrack’ is also associated with a commercial album that might be released for a film, and the entire soundworld of a film (dialogue, effects, music) is often referred to as its ‘soundtrack’.